If you are at a department store or mall, I recommend you to go to the information desk to give the lost item that you have found.
If you are on the street, it would be best to give the lost item to the police station. Although they may ask you to fill in a form, do not hesitate. You will have to give only simple information, such as your name, phone number and where you are staying.
If you are at a store and you find a lost item, give it to the cashier or a worker, who will keep the lost item until the owner comes to get it!